These studies are directed toward an understanding of cross-species grafts, using mouse and rat heart transplantation as models. Immunosuppressive and specific immunologic methods are being evaluated to achieve prolongation of graft function. Additional studies are directed toward physiologic aspects of canine heart grafts which are implanted in parallel with the recipient heart. BIBLIOGRAPHIC REFERENCES: Weil, Richard, III; Nozawa, Masumi; Weber, Collin; Koss, Michael; Chernack, William; McIntosh, Rawle; and Reemtsma, Keith: Cardiac Heterotransplantation: Morphological and Immunohistological Studies, Transplantation 19:150, 1975. Weil, Richard, III; and Reemtsma, Keith: The Mechanism of Xenograft Rejection, Proceedings of the First International Symposium on Liver Transplantation, Bonn, T.S. Lie, A. Gutgemann (Eds.) Verlag Gerhard Witzstrock GmbH, Baden-Baden, 1975, p.273.